Notable characteristics

Due to its extreme reactivity to air, this element always naturally occurs combined with other elements and compounds, as in the minerals strontianite, celestite, etc. It is isolated as a yellowish metal and is somewhat malleable. It is chiefly employed (as in the nitrate) to color pyrotechnic flames red.

Strontium is a bright silvery metal that is softer than calcium and even more reactive in water; strontium will decompose on contact to produce strontium hydroxide and hydrogen gas. It burns in air to produce both a strontium oxide and strontium nitride, but since it does not react with nitrogen below 380 °C it will only form the oxide spontaneously at room temperature. It should be kept under kerosene to prevent oxidation; freshly exposed strontium metal rapidly turns a yellowish color with the formation of the oxide. Finely powdered strontium metal will ignite spontaneously in air. Volatile strontium salts impart a beautiful crimson color to flames, and these salts are used in pyrotechnics and in the production of flares. Natural strontium is a mixture of four stable isotopes.

Strontium-90 has been used as a power source for RTGs. Strontium-90 produces about 0.46 watts of heat per gram, about 15% less power per gram then plutonium-238 which is the most common RTG fuel. Strontium-90 also decays roughly 3 times faster than plutonium-238. Most disadvantageously strontium-90 produces very little energy per volume (because of low density), making for large and bulky RTGs not suited for use in outer space. Its only advantages over plutonium-238 are that strontium-90 is much cheaper to produce (common waste product of nuclear reactors) and lacks negative public image (at least among those too young to remember milk scares involving strontium-90).

Occurrence

Strontium commonly occurs in nature, averaging 0.034% of all igneous rock and is found chiefly as the form of the sulfate mineral celestite (SrSO4) and the carbonatestrontianite (SrCO3). Of the two, celestite occurs much more frequently in sedimentary deposits of sufficient size to make development of mining facilities attractive. Strontianite would be the more useful of the two common minerals because strontium is used most often in the carbonate form, but few deposits have been discovered that are suitable for development. The metal can be prepared by electrolysis of melted strontium chloride mixed with potassium chloride:

Alternatively it is made by reducing strontium oxide with aluminium in a vacuum at a temperature at which strontium distills off. Three allotropes of the metal exist, with transition points at 235 and 540 °C. Strontium metal (98% pure) in January 1990 cost about $5/oz. The largest commercially exploited deposits are found in England.

Strontium was among the radioactive material released by the Windscale fire.

Isotopes

The alkali earth metal strontium has four stable, naturally occurring isotopes:
Sr-84 (0.56%), Sr-86 (9.86%), Sr-87 (7.0%) and Sr-88 (82.58%). Only Sr-87 is radiogenic; it is produced by decay from the radioactive alkali metal rubidium-87, which has a half-life of 48,800,000 years. Thus, there are two sources of Sr-87 in any material: that formed during primordial nucleo-synthesis along with Sr-84, Sr-86 and Sr-88, as well as that formed by radioactive decay of Rb-87. The ratio Sr-87/Sr-86 is the parameter typically reported in geologic investigations. Because strontium has an atomic radius similar to that of calcium, it readily substitutes for Ca in minerals. Sr-87/Sr-86 ratios in minerals and rocks have values ranging from about 0.7 to greater than 4.0.

Sixteen unstable isotopes are known to exist. Of greatest importance is Sr-90 with a half-life of 29 years. It is a by-product of nuclear fallout and presents a health problem since it substitutes for calcium in bone, preventing expulsion from the body. This isotope is one of the best long-lived high-energy beta emitters known, and is used in SNAP (Systems for Nuclear Auxiliary Power) devices. These devices hold promise for use in spacecraft, remote weather stations, navigational buoys, etc, where a lightweight, long-lived, nuclear-electric power source is required.

Precautions

In its pure form strontium is extremely reactive with air and spontaneously combusts. It is therefore considered to be a fire hazard.

The human body absorbs strontium as if it were calcium. Due to the elements being sufficiently similar chemically, the stable forms of strontium do not pose a significant health threat, but the radioactive strontium-90 can lead to various bone disorders and diseases, including bone cancer. The strontium unit is used in measuring radioactivity from absorbed strontium-90.
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